It is well known that spark ignition internal combustion engines will exhibit improved power and torque output, as well as reduced fuel consumption and exhaust emissions if such engines are equipped with variable valve timing operated with an optimized strategy.
With dual overhead camshaft engines, a first camshaft on each bank is usually operated either by single flexible, inextensible drive member such as a chain or belt or, alternatively, by gears from the crankshaft. With such a system, it is commonly known to shift the phase relationship between the crankshaft and both camshafts simultaneously by means of a belt-timing control system such as that shown in U.S. Pat. No. 5,088,456 to Suga, which is hereby incorporated by reference. The '456 patent discloses a type of so-called "dual-equal" system in which the intake and exhaust camshafts are phase shifted with respect to the crankshaft of the engine by the same amount. Although the dual-equal system will achieve certain benefits, it is generally incapable of changing the phase relationship between the camshafts of an individual cylinder bank. As will be explained below, it is desirable to have the capability to change the phase relationship between the dual camshafts on a cylinder bank.
U.S. Pat. No. 4,726,331 to Oyaizu discloses a system for changing the phase relationship between the intake and exhaust valves on a single cylinder bank of an engine. However, this system is incapable of changing the gross camshaft timing with respect to the crankshaft. More specifically, with the system of the '331 patent, the relationship between the exhaust camshafts and the crankshaft does not change. U.S. Pat. No. 5,109,813 to Trzmiel et al. discloses another system for altering the overlap between dual camshafts, with the system being responsive only to engine oil pressure. This system suffers from the deficiency that its dependency upon engine oil pressure renders it unable to control the camshaft phase relationship independently of engine speed. Although the concept of independently controlling the phase relationship between the crankshaft and both camshafts of a dual overhead camshaft setup is known, such a system would be very expensive and complex.
A system according to the present invention offers the advantage that the gross timing between both camshafts and the crankshaft can be controllably changed but, more importantly, the phase relationship between the two camshafts on a given bank of cylinders may also be controlled. This offers the important benefit of being able to change the valve overlap occurring between the opening of the intake valve and the closing of the exhaust valve. During operation at idle and very light loads, it is desirable to have little overlap between the opening of the intake valve and the closing of the exhaust valve. A small overlap is desired to minimize the fraction of the incoming intake charge which comprises exhaust gas remaining from the previous cycle because an excessive exhaust gas fraction will cause combustion instability at idle and, therefore, higher exhaust gas hydrocarbon emissions. On the other hand, at part loads, it is desirable to use exhaust gas recirculation ("EGR"); for this reason, external EGR valves and hardware have been installed on engines. Of course, such hardware is expensive and requires extra maintenance to keep the EGR system in order. Accordingly, if the valve overlap could be increased such that the number of degrees between the opening of the intake valve and the closing of the exhaust valve is increased, internal EGR could be increased, thereby obviating the need for an external EGR valve and its associated plumbing. A system according to the present invention uses variable valve overlap so as to provide variable internal EGR while also allowing a form of dual-equal phaseshifting.